COL100 �Introduction to Computer Science

Lecture 17 – Strings

February, 2025

Semester II

2024-2025

Vireshwar Kumar, Aaditeshwar Seth

CSE@IITD

Strings

• String is a sequence of characters (case sensitive)

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• Letters, special characters, spaces, digits

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• Enclose in quotation marks or single quotes

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hi = "hello there"

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Concatenate Strings

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hi = "hello there"

name = "ana"

greeting = hi + " " + name

# “hello there ana "

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Type Casting Strings

stringNum = “5“

print(type(stringNum))

intNum = int(stringNum)

print(type(intNum))

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String as an Array of Characters

• Inbuilt function len() gives the length of the specified string

fruit = “banana”

print(len(fruit)) #6

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Forward Indexing

• Strings are indexed. Square brackets are used to perform indexing into a string to get the value at certain index/position
• Example: fruit=“banana”
• fruit[0] -> “b”
• fruit[1] -> “a”
• fruit[2] -> “n”
• fruit[3] -> “a”
• fruit[4] -> “n”
• fruit[5] -> “a”

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• This indexing process is called “Forward" Indexing

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Backward Indexing

• We can negative integers to perform “Reverse" Indexing.
• Last character of the string can be obtained using the index -1, which refers to (n-1)th element.

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• Example: fruit=“banana”
• fruit[-1] -> “a”
• fruit[-2] -> “n”
• fruit[-3] -> “a”
• fruit[-4] -> “n”
• fruit[-5] -> “a”
• fruit[-6] -> “b”

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Looping through String

fruit = “banana”

for i in range(0,len(fruit)):

print (fruit[i])

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fruit = “banana”

for letter in fruit:

print (letter)

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first = “krsd”

last=“ing”

for letter in first:

print (letter+last)

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Slicing Strings

• Slicing extracts from a string by specifying indices: [start, stop, step]

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• If step is not specified, [start, stop] then a value of 1 is assumed for step (i.e., step = 1 as default).

fruit = “watermelon”

print(fruit[:5]) # “water”

print(fruit[5:]) # “melon”

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s = “abcdefgh”

print(s[3:6]) # “def”

print(s[3:6:2]) # “df”

print(s[::]) # “abcdefgh”

print(s[::-1]) # “hgfedcba”

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Methods for Strings

• Conversion to uppercase (capital) letters

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• Checks if the letter is lower case or not

s = “abcdefgh”

capital_s = s.upper()

print(capital_s)

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fruit = "Watermelon"

if fruit[0].islower():

print (“The first letter is not capital”)

else:

print (“The first letter is capital”)

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Find Method

• Finding a letter in a string using the find() method.
• Output index is found at forward index.
• -1 (is not found)

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fruit = ”watermelon"

print(fruit.find(“m”))

print(fruit.find(“g”))

print(fruit.find(“e”,4))

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String Comparions

• Two strings can be compared using comparison operators

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s1 = “acol100”

S2 = “acol100”

print(S1 == s1)

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word1 = “hello”

word2 = “yellow”

if (word1 < word2):

print (word1 + ’ comes before ‘ + word2)

else:

print (word2 + ‘ comes before ‘ + word1)

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Strings are Immutable

• Strings are "immutable" i.e., they can not be directly assigned, or we cannot assign a character to an index.

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• But a string can be sliced and then concatenated.

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word1 = “hello”

word1[0]=‘y’ # error

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word2 = word1[1:len(word1)]

word2 = ‘y’+word2

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Additional examples

• Check for palindrome using loops
• Look up various inbuilt string functions and see how you can write them yourselves

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